The UK's most complete GLP-1 guide
Every Question Answered. Every Side Effect Explained.
- How GLP-1 medications work
- Mounjaro vs Wegovy compared
- Injection instructions
- Side effects & solutions
- What to eat on treatment
- FAQs answered
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Wegovy Weight Loss vs Mounjaro — What's the Difference?
Both Wegovy (semaglutide) and Mounjaro (tirzepatide) are prescription-only weekly weight loss injections licensed in the UK. They work through different mechanisms and showed different results in clinical trials. This guide compares both medications factually to help you understand how they differ, so you and your prescriber can discuss which might be suitable for your individual health situation. Neither medication is assured to work for everyone — results vary significantly between individuals, and both work best alongside a reduced-calorie diet and increased physical activity.
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Enter your height and weight below. Supports both imperial (stone & lbs) and metric (kg). Results include NHS category, healthy weight range, and ethnicity adjustments.
What Is BMI and Why Does It Matter?
Body Mass Index (BMI) is the most widely used screening tool for assessing whether your weight falls within a healthy range. Used by the NHS, GPs, and weight management services across the UK, BMI provides a quick snapshot of your weight relative to your height. The formula is simple: your weight in kilograms divided by your height in metres squared (kg/m²).
While BMI isn't a perfect measure — it doesn't account for muscle mass, bone density, or fat distribution — it remains the standard first step in identifying potential weight-related health risks including type 2 diabetes, cardiovascular disease, high blood pressure, and certain cancers.
The NHS also recommends adjusted thresholds for people of South Asian, Chinese, or Black African heritage — a BMI of 23 or above is classified as overweight, and 27.5 as obese. Our calculator includes this adjustment as a one-click toggle.
How Wegovy and Mounjaro Work — Mechanism Differences
Wegovy (semaglutide) is a GLP-1 receptor agonist — it activates one hormone pathway in the body. GLP-1 naturally signals fullness, slows digestion, and helps regulate blood sugar. Wegovy mimics and amplifies these signals, which may help reduce appetite and food cravings.
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways simultaneously. GIP (glucose-dependent insulinotropic polypeptide) also affects appetite and fullness. This dual mechanism is what sets Mounjaro apart from Wegovy's single-pathway approach.
Clinical trial results: In the STEP-1 trial, Wegovy at the maximum dose (2.4 mg weekly) produced average weight loss of approximately 14.9% of body weight over 68 weeks. In the SURMOUNT-1 trial, Mounjaro at maximum dose (15 mg weekly) produced average weight loss of approximately 20.9% of body weight over 72 weeks. However, individual results vary widely — some people lose significantly more, others less, and some experience minimal weight loss.
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Three simple steps to understand your weight and take action.
Enter your height and weight in our free calculator. Get your BMI, NHS category, healthy weight range, and ethnicity-adjusted thresholds instantly.
If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
Use our progress tracker to log your weight weekly, visualise your journey with charts, and stay motivated — all stored privately in your browser.
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Practical Differences — Doses, Side Effects, and Eligibility
Dosing: Wegovy starts at 0.25 mg weekly and escalates to a maximum of 2.4 mg over approximately 16 weeks. Mounjaro starts at 2.5 mg weekly and escalates to a maximum of 15 mg over approximately 20 weeks. Both are once-weekly injections you can give yourself at home.
Side effects: Both medications commonly cause nausea, diarrhoea, vomiting, and constipation — especially during dose increases. These typically improve over time. Serious side effects are rare but can include pancreatitis (severe stomach pain) and gallbladder problems.
Eligibility: Both are available to UK adults with a BMI of 30 or above, or 27–29.9 with weight-related health conditions like diabetes or high blood pressure. Your prescriber will assess which is appropriate for you based on your individual health profile, medical history, and preferences. Neither medication is "better" — suitability is individual.
- Dual GIP/GLP-1 receptor agonist
- Up to 22.5% body weight loss (SURMOUNT-1)
- Weekly injection, dose escalation over 20+ weeks
- Available via private prescription in the UK
- Doses: 2.5mg → 5mg → 7.5mg → 10mg → 12.5mg → 15mg
- GLP-1 receptor agonist
- Up to 15–17% body weight loss (STEP trials)
- Weekly injection, 5-step dose escalation
- NHS & private prescription available
- Doses: 0.25mg → 0.5mg → 1mg → 1.7mg → 2.4mg
Mounjaro vs Wegovy: Head-to-Head
A side-by-side comparison of the two most popular GLP-1 weight loss medications available in the UK.
| Feature | Mounjaro | Wegovy |
|---|---|---|
| Drug class | Dual GIP/GLP-1 agonist | GLP-1 agonist |
| Active ingredient | Tirzepatide | Semaglutide |
| Avg. weight loss | 15–22.5% | 12–17% |
| Key trial | SURMOUNT-1 (2022) | STEP 1 (2021) |
| Max dose | 15 mg weekly | 2.4 mg weekly |
| Escalation period | 20–32 weeks | 16–20 weeks |
| Administration | Weekly injection (pen) | Weekly injection (pen) |
| NHS availability | Private prescription only | NHS & private |
| Approx. private cost | £150–£250/month | £150–£300/month |
| Common side effects | Nausea, diarrhoea, constipation | Nausea, vomiting, diarrhoea |
Data sourced from SURMOUNT-1 and STEP 1 clinical trials. Costs are approximate and vary by provider. Always consult a prescriber.
Am I Eligible for Weight Loss Medication?
In the UK, eligibility for GLP-1 weight loss medications like Mounjaro and Wegovy is based on your BMI and overall health profile.
- BMI of 30 or above (obese)
- Age 18 or over
- No contraindicated conditions
- Committed to lifestyle changes
- BMI of 27 or above
- Plus type 2 diabetes
- Or high blood pressure
- Or obstructive sleep apnoea
Use our BMI calculator to check whether your BMI falls within the eligible range.
How We Built These Tools
Every calculator and guide on myweightcheck is built with accuracy and transparency in mind. Our BMI calculator uses the standard WHO formula with NHS-aligned category thresholds. Our GLP-1 projections are modelled on published clinical trial data from SURMOUNT and STEP studies.
All data stays on your device. We don't require sign-ups, and we don't store your health information on any server. Your progress tracker saves data locally in your browser using localStorage, so only you can see it.
We're based in the UK and our tools are designed specifically for UK users — supporting stone & lbs by default, referencing NHS guidelines, using NICE-approved medication data, and linking to GPhC-registered pharmacies.
Frequently Asked Questions
Common questions about BMI, weight loss medications, and our free UK tools.
Is Mounjaro more effective than Wegovy for weight loss?
Clinical trials show Mounjaro produced slightly greater average weight loss (approximately 20.9% vs 14.9% of body weight). However, individual results vary widely with both medications. Some people lose more with Wegovy, others with Mounjaro. Effectiveness depends on your personal response, not just the trial average. Your prescriber will discuss which suits you best.
Which weekly weight loss injection is available on the NHS?
Both Wegovy and Mounjaro are licensed in the UK for weight management. NHS access is through specialist tier 3 weight management services — availability and waiting times vary significantly by region. Your GP can refer you to your local service. Private access is available through UK-licensed prescribers without NHS referral.
Can I switch from Wegovy to Mounjaro or vice versa?
Yes, switching is possible, but it must be managed by your prescriber. There is no official conversion chart — approximate equivalences exist based on clinical practice, but any switch requires a clinical assessment and a new prescription. Never switch medications without guidance from your healthcare provider.
What are the main side effects of semaglutide weight loss treatment?
Wegovy commonly causes nausea, diarrhoea, vomiting, constipation, and stomach pain — especially during dose escalation. These often improve over time as your body adjusts. Inject site reactions, fatigue, and headaches also occur. Serious side effects are rare. Report any severe stomach pain (possible pancreatitis) to your GP immediately.
How do I check if I'm eligible for Wegovy or Mounjaro in the UK?
Use our free BMI calculator above to check your BMI. If it's 30 or above, or 27–29.9 with weight-related health conditions, you may be eligible. However, a prescriber must assess your full medical history, medications, and health profile before prescription. Your GP or a licensed online prescriber can confirm eligibility through a clinical assessment.
Ready to take control of your weight loss journey?
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